The present invention relates to a method for preventing brittle fractures of steel pipe structures, and more particularly a method for preventing brittle fractures of steel structures, such as pipe line and steel pipe structures constructed by welding steel pipes.
Generally in butt welding, large residual tensile stress is caused around weld lines in a direction parallel to the weld lines, and in some cases the stress reaches a value close to the yield point of the base metal. Also, various factors, such as angular distortion, dislocation, and blow holes, which concentrate the stress are very likely to occur near or in the welded portions, and it is almost impossible to eliminate these adverse factors completely.
Particularly in circumferential joints of the pipe lines, the above residual stress exists as a tension stress in the circumferential direction, and is superposed by the circumferential stress caused by the internal pressure of the pipe along the full length of the pipe line, or caused by the pressure from outside the pipe such as by the overlying ground or vehicles moving on the overlying ground, so that the circumferential joint portions are intermittently subjected to the peak of the circumferential stress.
Further, in case when a nozzle is to be connected by welding with a certain portion of the pipe line, it is known that strong concentration of stress appears in the nozzle joint portions.
And, when the end portions of steel pipes are butt-welded to other steel pipes, steel plates or flanges, high tensile stress is always caused in the circumferential direction, and it is impossible to avoid the tensile stress.
Particularly, in steel pipe structures constructed by welding steel pipes, where steel pipes of relatively large diameter are used as a main support for the frame and steel pipes of relatively small diameter are used as a branch support, tee (T) joints where the seam welded portions of the both steel pipes intercross with each other are considered to be most susceptible to brittle fracture.
Brittle fracture occurs when the following three factors exist. (1) presence of site of tensile stress, (2) presence of notches or defects and (3) shortness of steel toughness, and in the welded steel pipes notches and welding defects are expected to exist in their seam welded portions, and particularly in case of electric resistance welded steel pipes (ERW) and continuous welded steel pipes (CW), the welding defects take a planar shape so that the defects develop into notches, and in addition in the end portions of the steel pipes which are butt-welded there exists high tensile stress in the circumferential direction, and in particular when the toughness of the seamed or continuous welded portions is not enough, the above three factors responsible for brittle fracture initiation are now active and provide a condition under which the brittle fracture is most ready to occur.
In order to avoid the above condition, it may be considered to eliminate completely the welding defects along the full length of the seam of a welded steel pipe or continuous welded steel pipe, and to provide means of improving the steel toughness. But these measures require various complicated procedures and cause increase in the production cost of the steel pipes.
Therefore, it has been desired to eliminate factors which cause brittle fracture occurring at the joint portions of steel structures by means of simple procedures. All the proposals ever made, however, fail to satisfy the demand.